DETROIT  As the "cash-for-clunkers" program finishes its fourth week, the government is offering a voucher program that could let consumers take delivery later of new cars that are unavailable now.

The Transportation Department said Thursday that consumers who want to purchase a new car not yet on a dealer lot can still be eligible for the car rebate program. This helps dealers and automakers who are finding it difficult to keep some hot-selling vehicles at the dealerships because of the popular government incentives.

Transportation Secretary Ray LaHood said the changes will expand buyers' choices and keep production lines running. He calls it a "wildly successful program."

Before the changes, eligible car buyers could only choose from vehicles on a dealer car lot. Ford, Toyota and Honda have increased production to meet demand from the clunkers program.

"We want to make sure we're maximizing the program," she said before the voucher change was announced. "We're not asking for any more money, or asking to expand the program."

At the end of July, some carmakers said they were starting to run low on popular models that fit the cash-for-clunkers program. Miller says it's a good problem to have, but one the government should be working to address.

The program is "exceeding everyone's expectations," she said. Depleted inventories are "a wonderful problem to have. We haven't have a conversation like that in this area for a long, long time."

While sales were up in July, Edmunds.com says interest in the program is beginning to wane. And dealers are getting angry because many aren't getting reimbursed by the government quickly enough, says Jeremy Anwyl, CEO of Edmunds.

"I think the bubble has kind of burst already," Anwyl said. Dealer showroom traffic is down 15% since the end of July, he said.

Fewer dealers may be inclined to participate after a Wednesday announcement from the National Highway Transportation Safety Administration, which oversees the program.

NHTSA added a notice to its Cars.gov site warning consumers not to sign any waivers that would force them to refund the dealer any money if the rebate falls through. And dealers have to allow buyers to take their new cars home, not leave them waiting at the lot for the credits to go through.

"Consumers are not required to sign contingency agreements to pay back the dealer should the CARS credit be rejected," the agency says.

Losing money on the deal is a big concern for many dealers, Anwyl said.

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